This invention relates to capacitive pressure sensors for use in industrial process control systems, and particularly to compensation of measurement error due to leakage conductance in such sensors.
Certain industrial process control transmitters employ capacitive pressure sensors and measurement circuits that measure industrial process variables. The measurement circuit includes a sigma-delta charge-to-digital converter and a processor that supplies the measurement value of the process variable for transmission to a central control station. In some embodiments, the sensor includes a metal diaphragm that serves as a common electrode for a differential pair of capacitive sensors. Different portions of the process variable are applied to opposite sides of the diaphragm to deflect the diaphragm based on the process variable. The capacitive sensors are charged by an input voltage, and the charge is transferred to the measurement circuit to derive the digital representation of the value of the process variable.
The sigma-delta circuit integrates the charges to increase or decrease an output signal over a number of sample cycles. The ratio of the number of steps of increase or decrease to the total number of samples represents the process variable value.
One problem with sensors of the class describe above is that leakage conductance generates measurement errors. Two common sources of leakage conductance include conductance across the sensor capacitor terminals, such as through a dielectric fill material (e.g., oil), and residual moisture on the circuit board forming the measurement circuit. Experiments reveal that a leakage resistance of about 1 gigaOhm (conductance as small as about 1 nanoSiemen) degrades the accuracy of a 16-bit digital signal to about 13 bits. Even where the excitation of the capacitive sensor is symmetric, the finite leakage still causes significant measurement error.
The present invention is directed to adaptive compensation of measurement error in industrial process control transmitters by which the process variable measurement value is corrected for error due to leakage based on the measured capacitance and leakage conductance of the sensor.
In one embodiment of the invention, an industrial process control transmitter is operated to compensate for errors in a process variable measurement due to leakage conductance in the transmitter. The industrial process control transmitter includes first and second capacitive sensors that sense the process variable and a measurement circuit coupled to the sensors that provides the process variable measurement based on ratio of the capacitance, e.g.,             C      H        -          C      L                  C      H        +          C      L      
where CH and CL are the first and second capacitors. A leakage conductance is measured for each of the first and second sensors. The process variable measurement is derived from the capacitance ratio and leakage conductances.
In preferred embodiments, first and second capacitance deviations, xcex4CH and xcex4CL, are identified for each of the first and second sensors based on the measured leakage conductance. First and second error expressions are derived based on ratios of the capacitance deviations to the total capacitance of the sensor. The process variable measurement is derived from the measured capacitance ratio and the first and second error expressions.
In some embodiments, the first error expression is             ϵ      1        =                  η        m            ⁢                                    δ            ⁢                          xe2x80x83                        ⁢                          C              H                                +                      δ            ⁢                          xe2x80x83                        ⁢                          C              L                                                            C            H                    +                      C            L                                ,
and the second error expression is             ϵ      2        =                            δ          ⁢                      xe2x80x83                    ⁢                      C            H                          -                  δ          ⁢                      xe2x80x83                    ⁢                      C            L                                                C          H                +                  C          L                      ,
where xcex7m is the measured capacitance ratio, and the corrected process variable measurement is calculated as xcex7m+xcex51xe2x88x92xcex52.
In some embodiments the values for the capacitance deviations are calculated during manufacture and stored in the processor for calculation of the corrected measurement. In other embodiments, the processor includes a look-up table that contains values of the deviation capacitances based on various sample frequencies and/or measured conductance of the sensor.